(a) Field of the Invention
The present invention relates to a cathode ray tube (CRT) and more particularly, to a cathode ray tube capable of effectively deflecting electron beams and having increased strength against external stress.
(b) Description of the Related Art
A CRT is a device for displaying image on a screen by vertically and horizontally deflecting electron beams generated from an electron gun and landing the deflected electron beams onto the phosphor layers formed on the screen. The deflection of the electron beam is controlled by a deflection yoke mounted on an exterior surface of a funnel of the CRT and which forms vertical and horizontal magnetic fields. The CRTs are generally employed for color televisions (TVs), monitors and high definition televisions (HDTV). And with the increasing use of the CRTs, there is a need to reduce the length of the CRT for increasing the brightness of the displayed image and for reducing the size of the final products, such as TVs, monitors and HDTVs.
In a CRT with reduced length, the electron beams should be deflected with wider-angles, and the deflection frequency and current supplied to the deflection yoke should be increased for the wider-angle deflections of the electron beams. As the deflection frequency and current increases, the deflection magnetic field tends to leak to the outside of the cathode ray tube and the power consumption increases.
In order to decrease the magnetic field leakage, a compensation coil is generally mounted with the deflection yoke. When, however, the compensation coil is employed, the power consumption of the cathode ray tube more increases. Alternatively, in order to decrease the deflection power consumption and the magnetic field leakage, it is conventionally preferable to decrease the neck diameter of the cathode ray tube and the outer diameter of the funnel near the neck side on which the deflection yoke is mounted, so that the deflection field efficiently acts on the electron beams. When the neck diameter simply decreases, there are disadvantages that the resolution of the image deteriorates due to the reduced diameter of the electron gun, and the outer electron beams are likely to be bombard the inner wall of the funnel, thus results in that the bombarded electron beams are not properly landed on the phosphor layer of the screen.
In order to solve these problems, U.S. Pat. No. 3,731,129 discloses a funnel having a wider peripheral portion sealed to the periphery of the panel, and a deflection portion whose cross-sectional configuration gradually varies from a rectangular shape substantially similar to that of the rectangular image produced on the panel to a circular shape. Thereby, the vertical and horizontal coils of the deflection yoke are closely located to the passage of the electron beams, and deflect the electron beams with reduced deflection power and without bombarding the electron beams to the inner wall of the funnel.
However, if the funnel having rectangular cross-section is designed without considering the external stress, such as the external pressure exerted onto the vacuumed funnel, the funnel does not have enough strength against the compressive stress produced by external pressure exerted on the vertical and horizontal directions of the rectangular shaped funnel and against the tensile stress produced by the external pressure exerted on the diagonal directions of the rectangular shaped funnel. Thus, it is difficult to form the funnel of rectangular cross-section having enough strength to endure against the external stress.